The present invention generally relates to an implant intended to be implanted between bony surfaces of a patient. Such an implant may replace a defective and painful joint or bone.
According to a particular application of the invention, the implant is used for treating ailments of the scaphoid (os scaphoideum).
The scaphoid is one of the eight or nine principal bones forming part of the wrist in humans. The wrist is a very complicated joint since it consists, not of two bones rubbing against one another, but of eight or nine principal bones of unusual shapes held and moving in equilibrium under control of a highly developed system of ligaments. The scaphoid is of particular importance clinically because it is the wrist bone which tends most often to be fractured.
The scaphoid is the largest of the bones located in the first row of wrist bones (known as the first carpal row or proximal carpal row). The other principal bones in the proximal carpal row are the lunate, triangular and pisiform bones. The bones of the proximal row are articulated to the radius (of the forearm) and the articular disk. The second carpal row (also known as distal row) contains the trapezium or greater multangular, the trapezoid or lesser multangular, the capitate and the hamate. The bones of this second carpal row are firmly attached to the metacarpal bones of the hand.
The scaphoid is surrounded by the trapezium, trapezoid, capitate, lunate and radius, as illustrated in FIG. 1 (which illustrates the bone of the right hand viewed looking towards the palm). The scaphoid is xe2x80x9carticulatedxe2x80x9d at the proximal side with the radius and at the distal side with the trapezium and trapezoid. Fractures of the scaphoid tend to occur, in around 70% of cases, in the central third thereof, as illustrated by the shaded area F in FIG. 1. If the fracture is not properly treated then a pseudathrosis or necrosis of the proximal bone fragment can occur. This is because, in a third of cases, blood supply to the scaphoid is furnished only by vessels at the distal side. Other ailments too, besides fractures, can lead to damage to or degeneration of the scaphoid.
U.S. Pat. No. 4,936,860 discloses a metal scaphoid prosthesis having a ledge engaging the trapezium and a bore receiving a suture to fix the prosthesis to the tendon slip or the palmar ligaments. Such attaching means aim to stabilize the prosthesis in the patient wrist, i.e. to force the prosthesis to return to its original position after a movement effected by the patient. This prosthesis has the drawback that complicate operations have to be carried out in order to appropriately fix it to the neighboring bones and ligaments. Furthermore, the material used to manufacture the prosthesis, namely vitallium, could wear away bones during movements made by the patient, as explained below.
It is not actually straightforward to determine the appropriate combination of shape, size and material enabling a suitable prosthesis to be produced for use in treating ailments of a joint or a bone. For example, the present inventors have found that there are disadvantages involved in the use of certain materials, known in the field of prostheses, such as polyethylene, ceramic zircon, titanium and vitallium.
In the case of polyethylene, the prosthesis is too soft, i.e. its modulus of elasticity, also called Young""s modulus, which is of the order of 1 GPa (Giga Pascal), is far too low compared with that of bone, the latter being comprised within the range of 15 to 25 GPa. Such a prosthesis is therefore subject to deformation and becomes crushed after a relatively short period of use. Wear debris resulting from deterioration of the prosthesis can then migrate in some areas of the patient""s wrist, which can cause painful inflammatory reactions for the patient.
In the case of zircon, titanium and vitallium, the prosthesis is too hard, so that it does not deform enough upon motions of the wrist, causing the bones in contact with the prosthesis to be stressed by the latter. Because of bad distribution of stresses, the patient experiences discomfort, for example, when pressing a fist down onto a surface. Also, there is a significant risk of wearing out the cartilage or bony surfaces in contact with the prosthesis due to the hardness of the prosthesis material.
It is an object of the present invention to provide an implant or prosthesis which resists wear and does not wear out the bones in contact with the prosthesis.
To this end, there is provided an implant having at least one contact surface portion designed to be in mobile contact with at least one bony surface when said implant is implanted in a patient, wherein said at least one contact surface portion is made of a material comprising pyrolytic carbon.
By xe2x80x9cmobile contactxe2x80x9d, it is meant that the at least one contact surface portion of the implant may rub, slide or roll on the at least one bony surface of the patient.
Thus, the implant according to the invention uses pyrolytic carbon to form a contact surface which will be mobile with respect to the neighboring bone(s). Pyrolytic carbon is known in the art for its property of biocompatibility. It is already used in the manufacture of prostheses intended to be fixed to a bone. In such prostheses, the pyrolytic carbon surface in contact with the bone is generally porous, in order to allow the bone to fill the pores and, in this manner, to reinforce its join with the prosthesis.
Unlike known prostheses, the implant according to the present invention uses pyrolytic carbon to rub against neighboring bones, and not to form a joining surface. The present inventors have discovered that pyrolytic carbon exhibits a good coefficient of friction with bone, so that neither the implant nor the bones with which it is in contact are worn away during movements made by the patient. The implant may thus roll or slide on neighboring bones without entailing damage. Furthermore, the use of pyrolytic carbon improves the stability of the implant, by allowing a smoother fit between the implant and the bones, so that there is little chance that the implant be ejected from its housing during movement by the patient.
A main reason why pyrolytic carbon behaves well in its function of rubbing with the bones, resides in the fact that this material exhibits a modulus of elasticity, also called Young""s modulus, approximately between 10 and 35 GPa, which is a substantially similar range to that of bone. Thus, when the implant presses against neighboring bones, and vice-versa, the respective stresses received by the implant and the bones equal each other out. The bones therefore receive stresses of the same order as those intended by nature. If the implant was, to the contrary, made from a material harder than bone, it would be likely to wear away or even perforate the neighboring bones. A too soft material would change the nature of the neighboring bones, making them loose their substance by decalcification. The bones would in this case wear away very quickly.
Accordingly, although pyrolytic carbon is the preferred material for manufacturing the implant according to the invention, other biocompatible materials could be used in the place of pyrolytic carbon, provided that their modulus of elasticity is approximately between 10 and 35 GPa.
According to another particularly advantageous feature of the invention, the external surface of the implant is polished in order to reduce even more the coefficient of friction. The external surface is preferably essentially made of pyrolytic carbon. Alternatively, the whole implant is made of pyrolytic carbon.
It is another object of the present invention to provide an implant or prosthesis which may be implanted into a patient more easily than the known implants.
To this end, the implant according to the invention is free from any attaching means, so that said implant remains free with respect to the neighboring bony surfaces when implanted in the patient.
The present inventors have discovered that it is possible, by virtue of the use of a material having a Young""s modulus approximately comprised between 10 and 35 GPa, to produce an implant which remains stable in the patient and does not entail any or much wearing away, without it being necessary to provide attaching means to attach the implant against a bone or a ligament. Once introduced into the patient, the implant remains substantially mobile in its housing between neighboring bony surfaces. This implies greater comfort for the patient, since by leaving the implant free with respect to the bony surfaces, the patient is permitted a greater degree of liberty of movement.
If the natural housing between the bony surfaces is not adapted to the shape and movements of the implant, the implant""s housing is prepared in advance by trimming one or several of the bony surfaces in an appropriate manner.
The implant according to the invention can be given a simple shape, since, on one hand, no attaching means is provided on its external surface, and, on the other hand, the housing into which it is to be introduced is prepared in advance so that its shape is adapted to the shape and movements of the implant.
The implant according to the present invention may be used notably as a joint between two bones or as a prosthesis intended to replace a bone or a part of a bone.
Thus, for example, when a joint between two bones has become painful, the facing bony surfaces, or only one of them, are trimmed to create a housing for the implant. The implant is then placed in its housing, to act as an interposition and joint piece between the two bones.
In the case where the implant according to the invention is used as a prosthesis, the defected part of the bone is removed, then a housing whose shape is adapted to the shape and movements of the implant is prepared by trimming one or several bones adjacent to the bone or the part of bone to be replaced. The implant is then introduced into the housing without being attached in order to restore the functionality of the defected bone.